A sturdy backpack with lots of storage: One of the unfortunate/awesome realities of Comic-Con is that you accumulate a lot of stuff. From the seemingly endless merchandise booths to the freebies handed out on the street, you'll need a place to store it all (and your other supplies) without breaking your back. My current con bag is the Herschel Supply Co. Little America backpack ($100), which has laptop/charger storage, padded straps, and front pockets for easy access.
Sunscreen: Just . . . trust me on this one.
A portable phone charger: Between keeping yourself occupied during long lines, navigating the con map, and taking cool photos, your phone battery tanks pretty quickly at fan conventions! Bring along a portable charger to keep your phone alive and your spirits high — without having to battle other fans over a few scattered wall outlets.
A general idea of your schedule: It's always prudent to take a long look at the convention schedule and prioritize which events you want to attend, where they're located, and how long the lines might be. However, be flexible — you probably won't have time for everything you want to see, so don't freak out if everything doesn't go according to plan!